In the .NET Framework, AppDomains provided a secure, isolated environment for applications to run within a single process, enabling dynamic assembly loading and unloading without impacting the entire application. With the advent of .NET 5 to 8, the focus shifted towards cross-platform compatibility and microservices, leading to the deprecation of AppDomains in favor of AssemblyLoadContext for assembly management and containers for application isolation. This transition reflects modern development practices, emphasizing performance, scalability, and compatibility across different platforms. Understanding these changes is crucial for developers migrating from the .NET Framework to newer .NET versions, as it affects application structure and deployment.